1. The Field of the Invention
The present invention relates generally to thermoplastic films. Specifically, the invention relates to stretched thermoplastic films with visual effects created by voiding agents.
2. Background and Relevant Art
Thermoplastic films are a common component in various commercial and consumer products. For example, grocery bags, trash bags, sacks, and packaging materials are products that are commonly made from thermoplastic films. Additionally, feminine hygiene products, baby diapers, adult incontinence products, and many other products include thermoplastic films to one extent or another.
Thermoplastic films have a variety of different strength parameters that manufacturers of products incorporating a thermoplastic film component may attempt to manipulate to ensure that the film is suitable for use its intended use. For example, manufacturers may attempt to increase or otherwise control the tensile strength, tear resistance, impact resistance, and breathability of a thermoplastic film. One way manufacturers may attempt to control or change the material properties of a thermoplastic film is by stretching the film. Common directions of stretching include “machine direction” and “transverse direction” stretching. As used herein, the term “machine direction” or “MD” refers to the direction along the length of the film, or in other words, the direction of the film as the film is formed during extrusion and/or coating. As used herein, the term “transverse direction” or “TD” refers to the direction across the film or perpendicular to the machine direction.
Common ways of stretching film in the machine direction include machine direction orientation (“MDO”) and incremental stretching. MDO involves stretching the film between two pairs of smooth rollers. Commonly MDO involves running a film through the nips of sequential pairs of smooth rollers. The first pair of rollers rotates at a speed less than that of the second pair of rollers. The difference in speed of rotation of the pairs of rollers can cause the film between the pairs of rollers to stretch. The ratio of the roller speeds will roughly determine the amount that the film is stretched. For example, if the first pair of rollers is rotating at 100 feet per minute (“fpm”) and the second pair of rollers is rotating at 500 fpm, the rollers will stretch the film to roughly five times its original length. MDO stretches the film continuously in the machine direction and is often used to create an oriented film.
Incremental stretching of thermoplastic film, on the other hand, typically involves running the film between grooved or toothed rollers. The grooves or teeth on the rollers intermesh and stretch the film as the film passes between the rollers. Incremental stretching can stretch a film in many small increments that are spaced across the film. The depth at which the intermeshing teeth engage can control the degree of stretching. Often, incremental stretching of films is referred to as ring rolling.
In connection with stretching a film, manufacturers may add a voiding agent to the film. Upon stretching, the voiding agent can create voids in the film; thereby, producing a breathable film. Manufacturers commonly use relatively large amounts of filler (50% by weight) and/or heat the film to an elevated temperature during stretching when creating breathable or porous films.
In addition to allowing for the modification or tailoring of the strength and the breathability of a film, stretching of a film can also reduce the thickness of the film. Stretched films of reduced thickness can allow manufacturers to use less thermoplastic material to form a product of a given surface area or size. Reducing the gauge of a film; however, can make the film more transparent or translucent. Consumers commonly associate thinner films and/or transparent films with weakness; and thus, may be dissuaded to purchase stretched films. Manufacturers may add pigments, such as TiO2, to add either color or opacity to thinner films. Unfortunately, additives, such as TiO2 can be expensive and often negatively impact the film strength properties, especially as the additive concentration is increased. Furthermore, even pigmented films commonly become less opaque upon stretching.
One common use of thermoplastic films is as bags for liners in trash or refuse receptacles. It is often undesirable to use porous or breathable films in trash bags as the voids in porous films may allow odor and/or liquids to escape from the bag. Additionally, many consumers may prefer opaque and non-transparent trash bags that prevent others (i.e., neighbors) from viewing the contents in the trash bag.
Another common use of thermoplastic films is as flexible plastic bags for storing food items. Similar to trash bags, in some instances it may be undesirable to use porous or breathable films in food storage bags because the voids in porous films can allow air and/or germs to reach and spoil the food within food storage bag. In other instances, however, a breathable food storage bag may be desirable. For example, a breathable food storage bag may be desirable when storing fruit and/or vegetables.
Accordingly, there are a number of considerations to be made in thermoplastic films and manufacturing methods.